I think this build is solid in general, but I guess I don't cap out on crit until I'm lower HP, so maybe that's a problem? Is attack speed more important than more damage once I have a full inv? Should I go 2x bloodthirsters in general? Should I take a defensive item?

Lol yeah, just curious, what do you use to basically say GG to the other team, because at this point, you're one of the strongest carrys in the game if you have 6/6 items, so those item choices really matter.

Usually at this point i have enough damage and crit with runs and items that i should be able to 3 and 4 shot anyone so i usually go with another phantom dancer for run and attack speed. This puts me over 2.0 attack speed and around 480 run speed...no one gets away anymore =P

Mercury Treads/Berzerker's Greaves (Based on their team)
Infinity Edge
Phantom Dancer
Phantom Dancer
Bloodthirster
Bloodthirster/Black Cleaver/Frozen Mallet (If they burst down a little too hard or if they have armor, usually they don't)

I pack cleanse as a summoner spell so I can buy just more damage items in place of quick silver sash. It's better in some cases and worse in others, but I think it works well enough. This build also gives you 100% crit and enough movespeed to chase even a fleeing Kassadin. I'm also too lazy to activate steroids like Youmuu's Ghostblade (or I forget) so I avoid buying it.

So armor pen from last whisper doesn't trump another phantom dancer? You'd think with his passive, you would be at 100% crit with trynd anyway by the time you're in the middle of a big fight right? So PD is basically only 15% MS and 50% AS, with diminishing returns on that MS.

I dunno, what's the logic here? Armor pen synergies perfectly with any physical damage, crits included. The less mitigation they have, the more damage you deal ultimately.

I also hear that a lot of trynds use crit runes... which would mean you have even less armor pen naturally, and you would be capped out on 100% crit a lot more reliably / earlier in the fight. All this in mind, you still recommend another PD over LW or something else like black cleaver?